The present invention relates to dual layer forming fabrics for use in papermaking, cellulose and similar machines.
Dual layer forming fabrics have only one set of machine direction yarns which bind two layers or sets of cross machine direction yarns. Each set of cross machine direction yarns is woven with a different interlacing pattern, prominent on a different side of the fabric, referred to as the sheet side and machine side of the fabric. The total width of the machine direction yarns, in relation to the total width available, referred to as machine direction cover, is usually more than 80%. The cross machine direction yarns occupy different layers. The cross machine yarns are vertically stacked so that in the case of there being an equal number of yarns in both sets, the projections of two adjacent sheet and machine side cross machine direction yarns on a horizontal plane usually overlap nearly completely. In the case of an unequal number of cross machine direction yarns in each set, this applies only for the cross machine direction yarns where their number is lower since they are not all stacked.
Dual layer papermakers' forming fabrics are manufactured in two basic ways to form an endless belt. First, they can be flat woven by a flat weaving process with their ends joined by any one of a number of well known methods to form the endless belt. Alternatively, they can be woven directly in the form of a continuous belt by means of an endless weaving process. Both methods are well known in the art and the term "endless belt" as used herein refers to belts made by either method. In a flat woven papermakers' fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross-machine direction. In a papermakers' fabric having been woven in an endless fashion, the warp yarns extend in the cross-machine direction and the filling yarns extend in the machine direction. As used herein the terms "machine direction" and "cross-machine direction" refer respectively to a direction corresponding to the direction of travel of the papermakers' fabric on the papermaking machine and a direction transverse this direction of travel.
Dual layer fabrics exhibit many advantages including an increased rigidity, extended life, improved sheet formation and mechanical stability. Even with the dual layer fabrics, however, marking has been a problem. The structure of the yarns, and/or the irregular mesh size leaves traces in the paper sheet in the form of a so-called wire marking. Early dual layer fabrics had a geometrical structure that made it impossible in practice to bring to a common plane the two yarn systems closest to the material to be formed. The difference in levels between the knuckles of the warp and weft yarns caused such a pronounced marking that these wires were useful only in forming coarse quality paper. Although with dual layer fabrics there is an improvement in wear resistance, it is generally not as much as one might expect. No known dual layer fabrics have achieved a geometry where the minimum distance of the machine direction yarns from the tangential plane of the machine side of the fabric, referred to as the machine direction yarn burial, was equal to or greater than the diameter of the machine side cross machine direction yarn. This geometry forms a fabric having what is referred to as "non-machine direction wear" condition.